Epacadostat is a novel inhibitor of indoleamine-2,3-dioxygenase-1 (IDO1) that suppresses systemic
tryptophan catabolism and is currently being evaluated in ongoing clinical trials. We investigated the effects of
epacadostat on (a) human dendritic cells (DCs) with respect to maturation and ability to activate human
tumor antigen-specific cytotoxic T-cell (CTL) lines, and subsequent T-cell lysis of
tumor cells, (b) human regulatory T cells (Tregs), and (c) human peripheral blood mononuclear cells (PBMCs) in vitro. Simultaneous treatment with
epacadostat and IFN-γ plus
lipopolysaccharide (LPS) did not change the phenotype of matured human DCs, and as expected decreased the
tryptophan breakdown and
kynurenine production.
Peptide-specific T-cell lines stimulated with DCs pulsed with
peptide produced significantly more IFN-γ, TNFα,
GM-CSF and
IL-8 if the DCs were treated with
epacadostat. These T cells also displayed higher levels of
tumor cell lysis on a per cell basis.
Epacadostat also significantly decreased Treg proliferation induced by IDO production from IFN-γ plus LPS matured human DCs, although the Treg phenotype did not change. Multicolor flow cytometry was performed on human PBMCs treated with
epacadostat; analysis of 123 discrete immune cell subsets revealed no changes in major immune cell types, an increase in activated CD83+ conventional DCs, and a decrease in immature activated Tim3+ NK cells. These studies show for the first time several effects of
epacadostat on human DCs, and subsequent effects on CTL and Tregs, and provide a rationale as to how
epacadostat could potentially increase the efficacy of immunotherapeutics, including
cancer vaccines.